Size-dependent bistability in multiferroic nanoparticles

Abstract

Most multiferroic materials with coexisting ferroelectric and magnetic order exhibit cycloidal antiferromagnetism with wavelength of several nanometers. The prototypical example is bismuth ferrite (BiFeO3 or BFO), a room-temperature multiferroic considered for a number of technological applications. While most applications require small sizes such as nanoparticles, little is known about the state of these materials when their sizes are comparable to the cycloid wavelength. This work describes a microscopic theory of cycloidal magnetism in nanoparticles based on Hamiltonian calculations. It is demonstrated that magnetic anisotropy close to the surface has a huge impact on the multiferroic ground state. For certain nanoparticle sizes the modulus of the ferromagnetic and ferroelectric moments are bistable, an effect that may be used in the design of ideal memory bits that can be switched electrically and read out magnetically. arXiv:1812.08297v4 [cond-mat.mtrl-sci]

Confined Magnons

“…Table II gives numerical estimates for the bulk and confined magnon resonances for antiferromagnets MnF 2 and FeF 2 , and for nanoparticles of room temperature multiferroic BiFeO 3 (assuming the AFM order is homogeneous instead of cycloidal). 9,27 The confined magnons are well below the bulk modes in all these cases so they should be observable with spectroscopic probes in the THz frequency range.…”

“…The nanoparticle magnetic and ferroelectric moments become bistable, enabling the design of ideal memory bits that can be switched electrically and read out magnetically. 27 While surface anisotropy has been measured in ferromagnets with scanning tunneling microscopy 28 and Brillouin spectroscopy, 11 its impact on the magnetic excitations of antiferromagnets has not been studied to date. It is desirable to identify the spectroscopic signature of surface/interface anisotropy so that the impact of surrounding materials on the magnetic properties of nanostructures can be understood.…”

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

“…Theory from the group of de Sousa has newly addressed the importance of the cycloid propagation direction in relation to the surface of the nanoparticles (which could also be extended to the surface of a sufficiently thin film), within the spin‐current model. [ 280 ] This investigates the change of both ferroelectric and ferromagnetic order parameters due to size effects. Here, the difference between “proximity effects” and “edge effects” are described.…”